Please PM backend super administrator login credentials and provide a username of a user that is supposed to be displaying, but isn't so I can compare with a user that is displaying to see what could be wrong.

Have reviewed your install and it's purely a configuration problem. You've set your userlist to show users from the "Registered" usergroup and nothing else. The user that you want to display that isn't is in the "Expired" usergroup. It only shows users that are specifically in the usergroups selected. The parameter specifically states this. Please edit your userlist and adjust "Usergroups" parameter as needed (e.g. add the Expired usergroup to it).

If left blank, doesn't it suppose to show all groups, regardless of assignment if the field is populated?

No, you need to specifically select what usergroups you want to be visible on your userlist. It has always functioned this way.

So I went to the list, and it is now showing members with empty fields. I didn't want the list to do that, I want it to only show members where the field is populated, regardless of what group they are in.

They probably have a NULL value. Add an advanced filter of the following.

( c.`FIELD_NAME` != '' AND c.`FIELD_NAME` IS NOT NULL )

We'll need to improve the <> blank usage to automatically add the null for a later release.

forge.joomlapolis.com/issues/4883